Seasonal changes in brain function are triggered by fluctuations in photoperiod. Daylength regulates both neural birth and survival in birds, but seasonal plasticity in mammals is reported only for synaptogenesis and neuropeptide expression. Neurogenesis and gliogenesis occur in CNS of adult mice and rats, but evidence for photoperiodic control of cell birth in adult brain is lacking. We have found that short days significantly increase the labeling of neurons with 5-bromo-2'deoxyuridine (BrdU), a thymidine analog which marks cell birth, in the subependymal zone and dentate gyrus of adult Syrian hamster. Furthermore, we have observed testosterone affects the distribution of newborn cells in the dentate gyrus of adult Syrian hamster. We will establish whether this effect is attributable to photoperiodic regulation of cell birth, migration, or programmed cell death. The distribution of BrdU-immunoreactive (BrdU-ir) neurons at different survival times will indicate whether photoperiod regulates cell birth or migration. We will characterize the phenotype (neurons or glia) of cells whose birth or survival is regulated by day length and utilize in situ end labeling to determine whether day length regulates apoptosis. Furthermore, we will examine the influence of castration and androgen replacement in long or short days of these processes. These experiments will elucidate environmental regulation of brain plasticity and explore basic mechanisms regulating cell birth, migration, and death which may prove relevant to treatment of damaged or diseased nervous systems.